Flash-induced electric potential generation in wild type and L212EQ mutant chromatophores of Rhodobacter sphaeroides: Q BH 2 is not released from L212EQ mutant reaction centers

Abstract

The nature of the protonation reactions altered in the L212EQ mutant of photosynthetic purple bacterium Rhodobacter sphaeroides, in which glutamate at position 212 of the L-subunit (Glu L212) is replaced by glutamine, was examined through the flash-induced electric potential generation in chromatophores, measured at various pH values. The second flash-induced electrogenic phase of secondary acceptor quinone (Q B) protonation in mutant reaction centers was 2–3-times smaller than that in reaction centers of wild type. Furthermore, the electrogenic reactions of the cytochrome bc 1 complex were absent in the mutant at pH 7.6, but appeared at lower pH, with normal sensitivity to antimycin and myxothiazol. Hence, in L212EQ chromatophores ubiquinol generated in the reaction center fails to take up one of its two protons and remains bound to protein at neutral pH. At acid pH, the second proton is taken up and ubiquinol is released from the reaction center.